Horizontal grinders have recently been developed for grinding a wide variety of materials including green wood waste and construction demolition. These machines include a feed system that is adapted to feed the wide variety of materials to a grinding unit which is adapted to effectively grind the materials and includes a feed conveyor and a feed roller. The grinding unit typically includes a grinding drum, which is rotated and includes hammers or blocks, and screens that hold material such that it will be forced into contact with the grinding drum until ground to a certain size.
The productivity of the grinding machines is related to the ability to control the feed system to deliver the material to the grinding drum at a rate equal to the capacity to grind. If the material is not delivered to the drum fast enough, the rate of grinding will be less than the potential. If the material is delivered too fast the material can become trapped between the grinding drum and the screens thereby increasing the risk of plugging. During normal grinding, the load on the grinding drum will typically increase in proportion to the rate at which material is being ground. When plugging begins, the load increases at a faster rate, and may reach an overload state. For grinders powered by diesel engines, the grinding unit may become plugged to the point the grinding drum will stop rotating, with material trapped between the grinding drum and the screens. This condition is undesirable, as it is difficult and time consuming to remedy. For grinders powered by electric motors the amperage draw may increase sharply, possibly damaging the motor or transmission components, or causing excessive power costs related to these spikes in electrical demand.
The overload condition can develop quickly. The feed systems are typically operated at a speed just below where the operator believes the machine may plug, in order to maximize productivity. Thus it can be difficult for an operator to control the feed system to avoid plugging. Systems have been developed to monitor for this overload condition, and subsequently automatically control the feed system. One such system is disclosed in U.S. Pat. No. 5,881,959, which describes a system that monitors for an overload condition of the grinding drum or of the feed system. If such a condition is detected, the feed system is stopped and can be reversed to correct the overload condition.